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GEK1532

Introduction to optics to

understand the eye

Thorsten Wohland

Dep. Of ChemistryS8-03-06

Tel.: 6516 1248

E-mail: chmwt@nus.edu.sg

http://micro.magnet.fsu.edu/primer/anatomy/

components.html

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

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Total internal reflection

E

 F

n1 n2

n1 < n2

E

 F

n1 n2

n1 > n2

Total internal reflection (TIR) can happen only when light propagates in a

denseer medium and comes to a interface with a less dense medium:

Example: TIR happens from glass to air or water to air, but not from air to

water or air to glass.

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Interference describes the superposition of two or more em waves resulting

in an amplification (constructive interference) or an attenuation

(destructive interference) of the amplitudes of the em field and thus in

intensities.

Interference

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Revision: pigmentsPigment

Class

Compound

Type

Colors

Porphyrin chlorophyll green

Carotenoid carotene

and

lycopenexanthophyll

yellow,

orange,

redyellow

Flavonoid flavone

flavonol

anthocyanin

yellow

yellow

red, blue,

purple,

magenta

http://webexhibits.org/causesofcolor/1B.html

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Scattering and Pigmentation

Rayleigh scattering,

stronger for blue light

(note: sometimesRayleigh scattering is

refered to as Tyndall

scattering)Retina

Iris

 Absorption of red

and green light

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Yellow pigment (blueis absorbed)

Scattering Layer 

(RayleighScattering)

Scattering and Pigmentation

Pictures from Andrew Parker¶s³Seven Deadly Colors´ (Free Press)

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Whiteness or Silveriness

Snow, foam, chalk, paper all are examples of 

materials that scatter and reflect light in some

way at many surfaces. The resulting color inall cases is white.

This white though can vanish when the air 

spaces are filled with some liquid (see oily

paper, or wet chalk).

random structures -> matt appearance

Regular structures -> shiny appearance

(sometimes silverish)

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Interference colors

 As in bubbles interference can as well work in feathers and wings.

Insect wings shimmer in many colors due to interference. A special

example here is the peacock:

http://webexhibits.org/causesofcolor/15C.html

One can see very fine branches on the

feathers. These are responsible for the multiple

reflections and the interference effects of thepeacock¶s feathers.

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Is it really interference?

Seen under different

angles the color of thepeacock feather changes:

a characteristic of 

interference colors

 Albino peacocks do not possessany melanin thus most white light

is reflected instead of absorbed.

The white light is much stronger 

than the interference color and

the peacock seems to be white.

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Violet as an interference color 

Ridges are 100 nm in size

White bar is 1/10 of a millimeter 

Structure in the wings responsible for the color 

Pictures from Andrew Parker¶s ³SevenDeadly Colors´ (Free Press)

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Examples of bioluminescence

UV light can be absorbed will then be given off at a longer wavelength.

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Snake, Benham¶s disk

http://www.michaelbach.de/ot/col_benham/index.html

Picture from Andrew Parker¶s ³SevenDeadly Colors´ (Free Press)

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The eye

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

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Mirrors, Virtual Images

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Mirrors, Virtual Images

For the eye it looks like the light

rays come from behind themirror.

We call this a virtual image

since the light does not reallyoriginate from the image.

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Spherical Mirrors

Seeing the light, Fig 3.6

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Optics: Lenses

f f 

xxOA

positive lenses:

Bi-convex

f f 

xxOA

negative lenses:

Bi-concave

Plano-Convex lens

Plano-Concave lens

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The function of lenses is based on

refraction

E

 F

n1 n2

n1 < n2

E

 F

n1 n2

n1 > n2

Snell¶s law: The difference between E and Fis the bigger, the bigger the

difference is between n1 and n2.

(n1sinE = n2sin F

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Lenses: Refraction

E1

 F1 E2 F2

na ng

na < ng (ref arctive indeces of air=1 and glass =1.5)

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Lenses: Refraction for a Plano

Convex lens

ng

na < ng (ref arctive indeces of air=1 and glass =1.5)

E1

 F1

na

E2

 F2

The same processes

happen as well at the Bi-

concave and Plano-

concave lenses

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Parallel incident rays will pass the

focal point after passing a lens «

f 

xx

f 

f 

x

f 

f 

xx

f 

Incident rays passing

the focus will be

parallel after passing

the lens.

« and we say the lens

has a focal length of 

³f´.

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Rays passing the center of the lens

pass undisturbed

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One ray will follow both rules: the

optical axis

OA

f 

x

f 

x

OA

f 

x

f 

x

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Image Formation

f f 

xx

OA

Object

Image

f f 

xx

OA

Object

Image

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Image Formation

f f xx

OA

Object

Image

Real images (all light rays are converging on the image) are upside down

compared to the object.

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Can we calculate the image

position?

f f xx

si

so

io s s f  

111

!Yes, with the so called

lens equation:

distanceimage

1

distanceobject

1

distanceocal

1!

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Can we calculate the size of the

image?

f f xx

si

so

o

i

 s

 s

M  !

distanceobject

distanceimageionmagni icat !

Magnif ication:

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Negative or concave lenses

f f 

xxOA

Parallel incidents rays

diverge and look like as if 

they would come from the

back focal point. The lens

has a focal length of ³-f´

f 

f 

xxOA

Rays thorough the

center of the lens pas

undisturbed.

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Image formation for a negative lens

f f xx

OA

Object

Image

Virtual images (the light rays are not converging on the image) are upright.

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Image formation for a negative lens

f 

f xx

OA

ObjectImage

io s s f  

111!

o

i

 s

 s!

si

so

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Example

oio s s s f  

2111!!

ios s f  

111!

oi s s !

When do we get an image that is equal in size to the object?

1!!

o

i

 s

 s

 f   s s io 2!!

Magnification is 1!

si

so

xf x

f xx

2f 

2f 

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The picture on the retina

The eye has a positive lens that creates a real picture on the retina.

One of the facts that was unimaginable for many people is that the

picture on the retina is as well upside down as we have seen for positive

lenses.

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Camera pictures

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

si

so

 A camera uses a lens (or a combination of lenses) to produce an image

on a film.

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Some special topics: Fresnel Lens

E1

 F1 E2 F2

na ng

Light is refracted at surfaces between two

materials. So only the surface of the lens is

the active part.

So can we reduce the weight of a lens and

thus the material needed?

Seeing the light, Fig 3.29

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Lens combinations

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

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 A single-lens reflex camera

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

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Summary

� Negative, positive lenses and description by lensequation

� Reversibility of light paths

� Lens combination (different back and front focallength)

� Image formation (real, virtual)

� Change in refractive index or lens curvature can

change focal length� Cameras (camera obscura, reflex camera)

� The eye, defects and corrections

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Remember this experiment

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Large slit: additive color mixing

between spectra from both sides

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 Additive primary colors mix to yield

subtractive primary colors